The present invention relates to a process for producing an acrylonitrile copolymer which is a precursor in the production of high quality carbon fibers. More specifically the invention relates to a process for producing acrylonitrile copolymer in an environment of less than 10 ppm of metal ions. The copolymer so produced is readily spun into fibers and then thermally converted into carbon fibers.
Carbon fibers prepared from acrylonitrile polymers and copolymers by a rapid oxidation process have superior physical properties such as increased tensile strength. The fibers are useful as reinforcement materials in automobile, aerospace, recreational and various other industries. An increasing demand for strong, lightweight materials insures an expanded use of carbon fibers in the future. Thus a need exists for a process which insures that the starting materials for producing carbon fibers are of the finest quality. A fine quality acrylonitrile polymer or copolymer has no defects such as voids formed when gases are expelled during fiber preparation. Also the fiber should not contain more than traces of metal contaminants, as these tend to degrade the fiber. The fiber should have a round shape for maximum stiffness.
Carbon fibers, which have heretofore been used as reinforcing material for plastic composite compositions, are preferably characterized by high tensile strength, high rigidity and a homogeneous fibrous structure. These characteristics can be adversely affected by certain properties found in the acrylonitrile copolymer feedstocks. If these undesirable properties can be identified and removed, then the final carbon fiber product is greatly enhanced in desirable characteristics.
U.S. Pat. No. 4,062,857 (Kobashi et al) discloses a process for preparing an acrylonitrile polymer in a substantially melted state. The polymerization, which is conducted under pressure, produces a polymer of uniform molecular weight with very little by-product. The patent does not exclude the use of vinyl sulfonic acid as a comonomer (Col. 2, line 65).
U.S. Pat. No. 4,049,605 (Kobashi et al) discloses a process for preparing an acrylonitrile polymer in a substantially molten state. The polymerization is conducted under increased pressure and in the presence of a catalyst such as a peroxide. The patent does not exclude the use of vinyl sulfonic acid or salts thereof as a comonomer (Col. 3, line 26).
U.S. Pat. No. 3,917,776 (Sato et al) discloses a process for producing a high quality carbon fiber. The feedstock can comprise an acrylonitrile copolymer containing units of acrylic acid, methacrylic acid or itaconic acid. Substantial amounts of a metal-containing catalyst are employed (claim 1).
The above-listed patents are incorporated herein by reference.
The process disclosed herein provides for the formation of acrylonitrile copolymer fibers having highest quality properties such as freedom from harmful sulfonic acid residues, freedom from more than 10 ppm metal ion contaminants freedom from voids, freedom from strong acids, suitably high molecular weight and increased stiffness.